Non-human primate bone marrow, cultured in the presence of mitogen-stimulated spleen cell supernatants gives rise to basophils. This suggests that the non-human primate basophil is the culture equivalent of the rodent mucosal mast cell. Human basophils of approximately 6% purity can be grown marrow using lectin-stimulated peripheral blood mononuclear cell supernatants. These cells are maximal in number at 2 weeks. Long-term cultures (6-8 weeks) give rise to small numbers of mast cells. IL-2 depletion of supernatants permits the growth of increased numbers of basophils. Mast cells appear in rheumatoid synovial tissues in association with T-helper cells and are ablated by interarticular injection of steroids. A rodent model of a mast cell mediated arthritis has been developed. The resultant arthritis is non-destructive and transient. These observations, taken together, suggest that mast cells are involved in arthritis and may play a role in exacerbations of disease activity. Primary microvascular endothelial cells produce both GM-CSF and a mast cell growth factor. Human mast cells produce both heparin and chondroitin sulfate E. The latter molecule had heretofore never been identified in human mast cells. The biologic role of chondroitin sulfate E is unknown.